Hybrid electric vehicles (HEV's) and fuel cell vehicles (FCV's) have been developed in an effort to provide a more efficient and environmentally friendly transportation option. In these vehicles, a power assist device is implemented to provide power for acceleration, to recapture electrical energy during regenerative braking and to store electrical energy. Traditionally, battery systems have been employed to provide these functions.
Super-capacitor's (SC) have been considered as an alternative to batteries to provide the above-described functions. An SC is an energy storage device that includes both battery and traditional capacitor characteristics. The SC can be charged and discharged quickly, but exhibits significantly enhanced capacitance over traditional capacitors. As a result, the SC can supply power for vehicle acceleration and/or for auxiliary electrical components and store energy generated during operation of the vehicle.
The state of charge (SOC) of an SC indicates the amount of usable energy stored within the SC at a given time. It is analogous to the amount of fuel in a fuel tank. In order to improve vehicle power management, it is beneficial to know the SOC of the SC at any given moment. The state of health (SOH) of an SC indicates the physical condition of the SC. The SOH is based on observing particular SC parameters to determine the extent of unobservable, internal damage or degradation. The power capability of the SC is useful in the optimal operation of a vehicle with more than one power source (e.g., a SC and an internal combustion engine), as the vehicle controller can allocate effectively the power requests to the various power devices.